The present invention relates to a solid oxide fuel cell.
A fuel cell is a power generation device having an anode (fuel electrode) and a cathode (air electrode) on both sides of an electrolyte, wherein a fuel gas is fed to the anode while an oxidizing agent gas is fed to the cathode to generate electricity by reacting the fuel and the oxidizing agent electrochemically by way of the electrolyte. A solid oxide fuel cell as a sort of fuel cells not only has high power generation efficiently but also is operated at a high temperature of from 600 to 1,000° C. Accordingly, as it is possible to reform the fuel in the fuel cell, the solid oxide fuel cell can increase the variety of fuel to be used. Furthermore, it can simplify the fuel system structure. Accordingly, it has an advantage of reducing the cost compared with other fuel cells. Naturally, since the exhaust is at a high temperature, it can be utilized easily and has a feature capable of easily forming not only heat-electricity combined system but also a hybrid system with other systems such as a gas turbine.
The fuel cell is generally classified into a cylindrical type and a plate type depending on the shape of a solid electrolyte. The cylindrical type is more resistant to thermal stresses compared with the plate type, and this is a remarkable advantage for SOFC which is operated at a high temperature.
However, the cylindrical type generally has a higher internal resistance compared with the plate type. One of the causes of high internal resistance is due to a long current path in the cylindrical cell. Further, it since also undergoes restriction upon connection of between unit cells, it has a problem that it is difficult to increase the volumic energy density. For overcoming such problems, it has been proposed for the improvement of making the cell-shape into flat tube-form (refer to JP-A No. 2005-166527).
In the cell described in the above-mentioned patent publication, inter connectors for taking out a current from cells takes up one-half surface of the cells because of serial and parallel connection of cells. Accordingly since the power generation area is decreased, so that the amount of power generation per unit cell can not be increased sufficiently. Accordingly, the volumic energy density can not also be increased sufficiently.
Then, in order to increase the power generation area, it may be considered a cell structure of providing a porous current-collecting electrode at the periphery of the anode making it possible to generate power on both surfaces of a flat tube cell.
However, in the cell provided with the porous current-collecting electrode, the current-collecting electrode becomes a bar for the supply of fuel or air to the cell surface, and it makes difficult to bring fuel or air up to the downstream of the cell. As a result, the electrode current cannot distribute uniformly on the downstream side (for example, in the axial direction), thereby the power generation efficiency is lowered.